The present invention relates to a highly sensitive image pickup apparatus which is capable of imaging even at a low illumination level and which is suitable, for example, for observing a star or a comet in a night sky.
An image pickup apparatus of this kind has conventionally comprised a combination of a highly sensitive television (TV) camera, and a noise reducer for improving the S/N ratio of a video signal delivered from the television camera. When a comet, which is observed only dimly in deep space by the naked eye, is picked up by a conventional highly sensitive color TV camera, a satisfactory video signal cannot be obtained from the TV camera. Therefore, it has generally been necessary to improve the S/N ratio of the video signal by means of the noise reducer.
The basic principle of a noise reducer is described below. A digitized input video signal is multiplied by a coefficient (1-k) by a multiplier, and a one-frame delayed video signal which has been delayed in a one-frame delay line is multiplied by a coefficient (K) by another multiplier. These multiplied signals are added by an adder and an added output signal thereby is further fed to the one-frame delay line. Thus, the input video signal is recursively processed through the one-frame delay line and multiplied by the coefficient (K) after being multiplied with the coefficient (1-K). As a result, the video signal having correlation and the noise having non-correlation are averaged and the S/N ratio is improved. In case a comet or a star is imaged (picked up) by a television camera, since the obtained video image can be considered to be virtually a stationary image and the noise has no correlation, the S/N improvement factor obtained can be expressed by the following equation: ##EQU1##
The above-described conventional highly sensitive image pickup apparatus has the disadvantage that it is necessary to use a relatively complicated and expensive noise reducer in combination with a television camera, and a video signal is not always improved satisfactorily by such noise reducer.